A reference system having inertial instruments rigidly fixed along a vehicle-based orientation such that the instruments are subjected to vehicle rotations and the instrument outputs are stabilized computationally instead of mechanically is termed a gimballess or strapdown system. Such systems generally include computing means, receiving navigational data such as magnetic and radio heading; air data such as barometric pressure, density, and air speed; and output signals of the inertial instruments for generating signals representative of vehicle position and orientation relative to a system of known coordinate axes, usually earth oriented. The presence of high angular rates associated with strapdown systems adversely effects performance and mechanization requirements. Consequently, such reference systems have been used extensively in missiles, space, and military vehicles, but their use in commercial aircraft has been less extensive because of economic constraints associated with the manufacture of precision mechanical assemblies, i.e., gyroscopes and other precision sensors.
Ballistic trajectories and projectile epicyclical motion result in angular rates and linear accelerations having frequency spectra from 0 Hz to approximately 10 Hz. When these signals are sensed by a strapdown inertial sensor in a spinning projectile, the sensed signal (rate or acceleration) is modulated by the spin frequency (F.sub.S). This results in the sensed signals having a frequency spectrum in the range of (F.sub.S -10) Hz to (F.sub.S +10) Hz. Multisensors have been used to separate rate and acceleration components by which one multisensor effectively measures two axes of angular rate and two axes of linear acceleration normal to the spin axis. Transducers in the form of multisensors such as these have been developed and used in aircraft and missile applications, being mounted on a spinning synchronous motor. Multisensors such as this have been described in U.S. Pat. No. 4,520,669 issued to Rider on Jun. 4, 1985 and assigned to Rockwell International Corp., the disclosure of which is incorporated herein by reference.
Standard strapdown inertial measuring technology applied to spinning projectiles (projectiles that spin at 100-350 revolutions per second) is impractical with available component technology. The primary limiting factors are as follows (1) available rate gyros (measuring angular rates such as roll, pitch, or yaw) cannot measure the high angular rates associated with a projectile spinning at 100-350 revolutions per second, (2) gyro scale factor errors may result in unacceptably large rate errors even when the high spin speeds can be measured, and (3) high centrifugal acceleration, in combination with mechanical misalignments, prevents accurate measurement of spin axis acceleration. Further, strapdown algorithms cannot be iterated at a high enough rate to accurately track the high spin speed.
Therefore, there is a need and desire for an artillery shell tracking system using a roll rate sensor, not limited by the high roll rates associated with spin stabilized projectiles. Further, there is a need and desire for a shell mounted low cost navigation system. Further still, there is a need and desire for an INS having improved accuracy by applying GPS measurements to provide error correction to INS attitude uncertainties. Further still, there is a need and desire for an INS having magnetic sensors to measure roll speed to despin a body axis frame measurements to a zero roll rate despun axis frame.
There is also a need and desire for a cost effective method of providing attitude, velocity, and position of a spinning projectile by utilizing a combination of inertial, magnetic and GPS measurements.